Merrimack climate

Merrimack Frost Dates and Growing Season

NOAA 1991-2020 normals for Nashua 2 Nnw, 8.3 km from Merrimack, elevation 135 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Nashua 2 Nnw. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

The last spring freeze typically comes around April 30 and the first fall freeze around October 12, a growing season of about 165 days.

Median at 36°F
last May 10, first October 2
Median at 32°F
last April 30, first October 12
Median at 28°F
last April 17, first October 24

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold10%20%30%40%50%60%70%80%90%
36°FMay 23May 19May 16May 13May 10May 8May 5May 3Apr 30
32°FMay 11May 6May 4May 1Apr 30Apr 27Apr 24Apr 22Apr 18
28°FApr 29Apr 24Apr 21Apr 19Apr 17Apr 14Apr 11Apr 8Apr 3
24°FApr 16Apr 11Apr 8Apr 5Apr 4Apr 2Mar 31Mar 29Mar 23
20°FApr 5Apr 2Mar 31Mar 29Mar 26Mar 23Mar 19Mar 16Mar 11
16°FMar 30Mar 26Mar 23Mar 19Mar 16Mar 14Mar 11Mar 8Mar 3

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold10%20%30%40%50%60%70%80%90%
36°FSep 21Sep 25Sep 28Sep 30Oct 2Oct 4Oct 6Oct 10Oct 15
32°FSep 30Oct 2Oct 5Oct 8Oct 12Oct 15Oct 18Oct 22Oct 28
28°FOct 8Oct 13Oct 18Oct 22Oct 24Oct 28Nov 1Nov 3Nov 7
24°FOct 22Oct 27Oct 31Nov 2Nov 5Nov 8Nov 11Nov 15Nov 20
20°FNov 3Nov 9Nov 12Nov 16Nov 19Nov 22Nov 25Nov 29Dec 3
16°FNov 14Nov 20Nov 24Nov 27Dec 1Dec 2Dec 5Dec 10Dec 16

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold10%20%30%40%50%60%70%80%90%
36°F162155151148145142138134128
32°F185178173169165162158154148
28°F209202198194190186182178172
24°F235226221217213209205201195
20°F259252246242237233229224216
16°F279272267263259255251245239

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
100% of years
24°F or colder
100% of years
20°F or colder
100% of years
16°F or colder
100% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F
6,566
Cooling degree days, base 65°F
661
Growing degree days, base 40°F
4,777
Growing degree days, base 45°F
3,681
Growing degree days, base 50°F
2,723
Growing degree days, base 55°F
1,899
Growing degree days, base 60°F
1,209

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,26900
February1,08800
March95307
April559360
May24427262
June54117513
July6245704
August12196649
September12168400
October4444110
November743016
December1,07301

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 8.3 km from Merrimack. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00275712 (Nashua 2 Nnw). Public domain.